A thermosensitive recording medium containing a thermosensitive color developing layer, the major component of which is an electron accepting color developing agent (henceforth referred to as “color developing agent”) that develops color when heated with a colorless or pale colored electron donating leuco dye (henceforth referred to as “dye”), is widely used. A thermal printer and the like in which a thermal head is contained is used to record on the thermosensitive recording medium. This recording method has many feature such as low maintenance, low-cost, compact size, clear color development, etc. as compared with other recording methods. Therefore, it is used extensively in facsimile machines, computer printers, automatic ticket vending machines, measurement recorders, handy outdoor terminals and the like. The thermosensitive recording medium is used not only as the output paper in the various devices mentioned above but is also becoming popular for use in applications such as vouchers and the like where excellent durability is required.
When a thermosensitive recording medium is used for various tickets, receipts, labels, bank ATM print outs, gas and electrical meter read outs and vouchers such as horse racing tickets and the like, the medium needs to have plasticizer resistance and oil resistance, to avoid the problem of printed letter readability when the medium is stored for an extended period of time in contact with a film or synthetic leather, and light resistance and heat resistance to prevent fading when the medium is exposed to sunlight for an extended period of time.
For these reasons, a thermosensitive recording medium prepared by using a diphenyl sulfone derivative as the color developing agent (References 1 and 2), a thermosensitive recording medium prepared with a phenolic condensation compound (Reference 3), a thermosensitive recording medium prepared with a phenolic compound (Reference 4) and also a thermosensitive recording medium in which other color developing agents are combined with these agents (References 5-8) and the like have been disclosed.    Reference 1: Japanese Patent Application Public Disclosure No. 2003-212841    Reference 2: Japanese Patent Application Public Disclosure No. H08-333329    Reference 3: International Publication WO 2002/098674    Reference 4: International Publication WO 2001/25193    Reference 5: International Publication WO 2005/087503    Reference 6: International Publication WO 2005/087504    Reference 7: Japanese Patent Application Public Disclosure No. H10-297089    Reference 8: Japanese Patent Application Public Disclosure No. H10-297090